The invention is in the field of capacitive voltage divider circuits, and relates more particularly to an improved high-voltage capacitive voltage divider circuit using a high-voltage Silicon-On-Insulator (SOI) capacitor.
In certain relatively high-voltage circuit applications, a circuit for generating a relatively low voltage proportional to the high voltage, and then using that low-voltage signal to control a circuit operation, is required. While various prior-art circuits for performing this function exist, they suffer from several significant drawbacks, particularly in the context of integrated circuit technology.
Thus, for example, a low voltage signal proportional to a sensed high-voltage signal can be generated by a simple resistive divider circuit. However, in such a circuit, the resistive divider will draw current, and thus dissipate power and generate heat, making such a solution particularly undesirable for integrated circuit applications. The power dissipation problem can be solved by using a capacitive divider circuit, but for relatively high-voltage integrated circuit applications, there has not heretofore been a capacitive component capable of being simply, economically and compactly formed in integrated circuit technology, thus requiring either a relatively large, expensive and uneconomical integrated circuit capacitive divider, or else the use of a discrete high-voltage capacitor, which has obvious drawbacks in terms of size, cost and complexity.
One example of a prior-art high-voltage capacitive divider circuit is shown in U.S. Pat. No. 5,485,292, having a common inventor with the instant application. In that reference, a high-voltage capacitor for use in a capacitor divider circuit is created by forming a plurality of series-connected capacitors using a dual-layer metallization with appropriate insulation formed on a field oxide layer. Although that configuration was an improvement over then-available constructions, by present standards the capacitor structure is relatively large, uneconomical and complex. Additionally, since capacitors must be connected in series to achieve the desired voltage breakdown level, each individual capacitor must be made larger than the total desired capacity to be achieved, thus further increasing the size and cost of the finished product.
One particular area in which a simple, compact and economical high-voltage capacitor divider circuit is required is in the area of Switched Mode Power Supply (SMPS) design. When SMPS circuits are designed to support universal mains, the AC input voltage will be in the range of 90-280 VAC. In order to provide under-voltage protection for such circuits, it would be extremely desirable to have a high-voltage capacitive voltage divider circuit which overcomes the drawbacks of prior-art circuits.
Accordingly, it would be desirable to have a high-voltage capacitive divider circuit which is simple and compact in construction, economical to manufacture, and which can be easily fabricated in integrated form.
It is therefore an object of the present invention to provide a high-voltage capacitive voltage divider circuit which is simple and compact in construction, economical to manufacture, and easily fabricated in integrated circuit form.
In accordance with the invention, these objects are achieved in a high-voltage capacitive voltage divider circuit having a high-voltage SOI capacitor connected between a high-voltage terminal and a low-voltage terminal, a low-voltage SOI capacitor connected between the low-voltage terminal and a common terminal, and circuitry for processing a signal at the low-voltage terminal.
In a preferred embodiment of the invention, the high-voltage SOI capacitor includes an oxide layer on a substrate, a thinned drift region on the oxide layer, a thick oxide layer over the thinned drift region, and an electrode layer over the thick oxide layer. In this configuration, the electrode layer and the thinned drift region form capacitor plates insulated from each other by the thick oxide layer.
In a further preferred embodiment of the invention, the capacitive voltage divider circuit forms part of an SMPS, the high-voltage terminal is an output of a diode bridge rectifier of the SMPS, and a comparator is used to sense a stepped-down voltage proportional to a voltage at the output of the diode bridge rectifier and uses the stepped-down voltage to detect an under-voltage condition in the SMPS.
A high-voltage capacitive divider circuit in accordance with the present invention offers a significant improvement in that a simple, compact, economical to manufacture and easily-integrated construction can be achieved.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.